PROJECT ABSTRACT Cancer immunotherapy targeting the adaptive immune system results in cures of previously considered terminally ill cancer patients. This remarkable achievement has excited basic researchers and clinicians alike to search for immunotherapies that can treat a higher percentage of patients and more cancers. We now know that an effective adaptive immune response to cancer depends on a robust innate immune response, and that Stimulator of Interferon Genes (STING) is the major innate immune pathway that sparks the anti-cancer immune cascade. The least understood molecule on the STING pathway is the second messenger 2?3?-cyclic GMP-AMP (cGAMP), which has been characterized as an intracellular signal. We discovered that cGAMP is also an extracellular signal, or immunotransmitter, that activates the anti-cancer innate immune response. This proposal aims to determine the mechanism of this novel extracellular cGAMP cell-to-cell signaling and develop tools to enhance it therapeutically. cGAMP is exported by cancer cells, and it can be imported by other cells to activate their STING pathway. Its extracellular concentrations are regulated by the dominant cGAMP hydrolase ectonucleotide pyrophosphatase phosphodiesterase 1 (ENPP1), which is an extracellular protein. This puts ENPP1 on the map as an innate immune checkpoint. I will probe the mechanism of extracellular cGAMP signaling by focusing on the enhancement of extracellular cGAMP by export and the suppression of extracellular cGAMP by ENPP1 degradation. Specifically, I will develop a small molecule ENPP1 inhibitor that can be used to test if ENPP1 is an innate immunotherapy target. Then, I will identify the dominant cGAMP export mechanism in cancer cells. This research bridges the interface between fundamental discovery and therapeutic development and will generate the pillars of a basic biochemical foundation on which to build new generations of immunotherapies. During this project, I will build my biochemical toolbox for dissecting molecular mechanisms in the immune response to cancer, and I will develop an intuition for asking important biological questions. Furthermore, I will communicate my science via proposals, presentations, and conversations, and will be involved in mentoring and teaching other scientists around me. Supported strongly by my sponsor, co-sponsor, and the colleagues and collaborators with whom I have forged relationships at Stanford University, I look forward to training towards becoming an independent group leader in the field of chemical biology.